Dyslexia Etiology: Genetic, Neurobiological, and Environmental Factors

Dyslexia is a specific learning disability characterized by persistent difficulties in accurate/fluent word recognition, decoding, and spelling despite adequate intelligence, education, and sensory function. It affects 5–15% of the population.

Genetic Factors

• High heritability (~40–60%); familial aggregation.
• Candidate genes: DCDC2, KIAA0319, DYX1C1, ROBO1, FOXP2—involved in neuronal migration, axon guidance, ciliary function, and language networks.
• Polygenic architecture; genome-wide studies identify multiple loci of small effect.

Neurobiological Correlates

• Structural: Reduced gray/white matter in left temporoparietal (TP), occipitotemporal (OT), and inferior frontal regions; altered corpus callosum.
• Functional (fMRI): Hypoactivation of left TP (phonological processing) and OT (visual word form area) during reading; compensatory right-hemisphere and frontal overactivation.
• Connectivity: Disrupted left arcuate fasciculus (phonological loop) and inferior longitudinal fasciculus (visual-orthographic).

Cognitive Mechanisms

• Phonological deficit (core): Impaired phoneme awareness, rapid naming, verbal working memory.
• Visual processing deficits (subset): Magnocellular pathway dysfunction → unstable visual perception, crowding.
• Procedural learning deficits: Impaired implicit skill automation (cerebellar/basal ganglia circuits).

Environmental Modulators

• Early language exposure, shared reading, print-rich environment → protective.
• Quality of reading instruction (explicit, systematic phonics) → mitigates severity.
• Socioeconomic status, bilingualism (complex interaction), stress/toxic stress.

Key Clarifications

• Dyslexia is not due to low IQ, laziness, vision problems (though co-occurring vision issues should be corrected), or poor teaching alone.
• It is a neurodevelopmental difference; early identification and evidence-based intervention (structured literacy) enable skilled reading.